Black plays exd4. Black is a pawn up but has a number of weaknesses the square e5, the Knight on f6 that can be pinned and also the weak f7 square. This all adds up to giving White an number of options. They are.

5. O-O the classical variation.

5. e5 the modern variation.

5. Ng5 a misguided direct attack on f7.

5. Nxd4 rather normal move.

5. Bg5 pinning the Knight, but the pin can soon be undone, leaving no advantage for White.

5. Qe2 this supports e5, although e5 can be played immediately anyway.

5. c3 fails to 5...Nxe4 where Black has the nice d5 available and time to block any pins on the e-file against the King.